I. Field of the Invention
This invention pertains to a process for heat treating a coating applied to an automobile body and, more particularly, to setting, drying and/or curing a fresh coating (such as paint or the like) applied to a newly completed automobile body.
II. Description of the Prior Art
During the manufacture of an automobile, it is desirable to provide a finished vehicle body having a high quality finish. The high quality of the finish improves the marketability of the vehicle as well as protects the vehicle body from the elements. In the automobile industry, it is recognized that not all surfaces of the automobile body need have the same quality of finish. For example, those surfaces of the automobile body which are not readily visible to the vehicle user do not require a high gloss finish but may only require a finish sufficiently good to protect the vehicle's surface from the elements. Recognizing the intricacies of an automobile body and the various requirements for the finished coat at different locations on the automobile body, the industry has developed certain nomenclature and standards which are used when referring to the finished coat of an automobile. For example, the industry has employed the phrase "Class A surfaces" which refers to those surfaces of the automobile body which are readily visible to an individual inspecting the automobile under normal conditions. These surfaces include the outer door panels, the fenders, the exterior hood, the exterior trunk and the top of the vehicle. Excluded from Class A surfaces are such surfaces of the automobile which are not seen during normal inspection of the vehicle. Such surfaces will include the inner walls of the door posts and side walls of door panels.
In addition to segregating the various surfaces of an automobile into identifiable classifications, the industry has established (in addition to other methods) a scale referred to as the "Tension scale" to provide for means of characterizing the quality of a finish on a given surface. The Tension scale is a measurement of the reflectivity of the surface. A very high quality and reflective surface will have a high scale number. The scale has a range of 0 to 20 with 20 being a mirror quality surface. In the industry it is recognized to be beneficial to maximize the Tension scale rating of Class A surfaces while insuring that non-Class A surfaces have a sufficient covering to protect the non-Class A surfaces from the elements.
In the prior art of providing finishes for automobile bodies in an automobile assembly plant, the automobile bodies would be intensely cleaned prior to receiving several coats of finish. A common first coat for an automobile body was an electro-deposition of a coating. This coat was commonly referred to as a "E-dip" or "E-coat". The E-coat normally took place in an apparatus where the automobile body was either charged or grounded and the coating to be applied was charged oppositely of that of the body.
Following the application of the E-coat, the automobile body would receive a second coat referred to as the "base coat" or "color coat". Historically, the color coat was a solvent based coating. After the color coat was applied, the automobile body was taken to a flash-off area where the color coat was allowed to set momentarily. After the color coat had partially set, the vehicle was then subjected to a clear coat which was often applied while the color coat was still wet. With the color coat and clear coat applied, the automobile body was admitted to an oven for a period of time to initially set the clear coat. This would commonly last about eight minutes. After leaving the oven, the vehicle was admitted to a hot air convection oven for a more extended period of time to cure the clear and color coats. Not uncommonly, the more extended period of time would last approximately 35 minutes.
With common assembly line speeds of 15 to 28 feet per minute, the area in an assembly plant devoted to setting and curing the automobile finish could be immense. For example, assuming an assembly line speed of 35 feet per minute and an oven time of 40 minutes, 1,200 feet of assembly line would be required to dry and cure the automobile body finish.
In addition to the amount of assembly line which would be devoted to finishing the auto body, the prior techniques of auto body finishing presented certain environmental and occupational hazards. For instance, the solvent based coatings would generate noxious vapors which presented concerns to both the safety of the workers and the environment. As a result of these potential problems, an interest has developed in using water based coatings as the color coat. However, simply substituting a water based color coat for a solvent based color coat complicates the finishing process because water based paints are typically slower to dry than the solvent based paints. Also, the application of a clear coat on top of a water based color coat is likely to lead to complications in the finishing process. Such complications include bubbling, cracking or "popping" of the finish as the water vapor of the color coat attempts to pass through the clear coat. This will occur when the water vapor of the color coat is not completely dried prior to application of the clear coat. This problem typically did not occur with the use of solvent based color coats since the solvents from the color coat are compatible with the clear coat and can pass through the clear coat.
From the foregoing, it will be appreciated there is an on-going need to reduce the amount of time necessary to effectively cure coats on an automobile body. This need is particularly acute in applications using water based color coats. The reduction in the amount of necessary time also results in a reduction of assembly line space which must be devoted to the finishing process. While reduction in the amount of necessary time to effect the finishing operation is an industry goal, this effort is constrained by the requirement of adequately coating the automobile body to protect its surfaces from the elements and to provide Class A surfaces having as high a quality of finish as possible.